bpf: Add the __arena and __nullable BTF type tags

Kernel structures increasingly want to publish pointers into a BPF arena
to the BPF programs they are shared with, and some pointer members that
reach programs as trusted are legitimately NULL at times. Neither can be
expressed today: a pointer member reaches the program as an ordinary
kernel pointer, and sched_ext resorts to a plain u64 holding a
pre-translated arena address that programs cast before use.

Define __arena and __nullable as BTF type tags, the way __user, __rcu
and __percpu already are, so that tagged members survive into vmlinux
and module BTF and the verifier can act on them. To kernel code and to
sparse the qualifiers are empty: the pointers are ordinary kernel
pointers. The verifier side follows in the next patches.

The tags live in a header of their own, bpf_compiler_types.h, rather
than in compiler_types.h. They mean something to the BPF verifier only,
and only the few structures shared with BPF programs that want to
publish such a pointer carry them, so the code that declares such a
structure includes the header and the rest of the kernel never sees the
qualifiers.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261019132203.3034698-6-memxor@gmail.com

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